Recent development in communication industry, the demand for the optical fiber with lower transmission loss and low manufacture cost has been increased. A plastic optical part has merits of design facility and low manufacture cost, compared with a glass optical part with identical structure, so the plastic optical part has been recently utilized as an optical fiber, an optical lens element, an optical waveguide and so forth. Especially, a plastic optical fiber (referred to as “POF”), entirely composed of a plastic is suitable for manufacture of the optical fiber with large diameter at a low cost, because the POF has advantages in excellent flexibility, light weight and high machinability, compared with the glass optical fiber. Accordingly, it is planned to utilize the plastic optical fiber as an optical transmission medium for short-distance purpose in which the transmission loss is small (for example, Japanese Laid-Open Patent Publication (JP-A) No. 61-130904).
The POF is composed of a core part formed from a plastic, and an outer shell (referred to as “clad” or “clad part”) that is formed from a plastic having lower refractivity than the core part. The POF is manufactured, for example, by forming a tubular clad part (referred to as “clad pipe”) by melt-extrusion, and by forming the core part in the clad pipe. A graded index (GI) type POF, in which the refractive index in the core part gradually decreases from the center to the surface of the core part, has high transmission band and high transmission capacity. Various methods for manufacture of the GI type POF are disclosed. For instance, U.S. Pat. No. 5,541,247 (counterpart of Japan Patent No. 3332922) describes a method to manufacture the GI type POF by forming an optical fiber base body (hereinafter referred to “preform”) by use of interfacial gel polymerization, and then by melt-drawing the preform in a heating furnace.
In order to increase the machineability of the POF, the preform is drawn by melt drawing under the condition in which the diameter of the POF is 100 μm or less and the drawing tension is 10 g or more, as described in JP-A No. 07-234322. Moreover, JP-A No. 07-234324 sets the drawing tension of 100 g or less in the melt-drawing process for the purpose of preventing decrease in the transmission loss that is caused by shrinkage of the heated POF.
In the manufacture of the POF, the POF is continuously wound around a bobbin, for example, for keeping the POF. Especially, storage of the rolled POF for a long time causes to increase the transmission loss of the POF even at the room temperature. Such problem cannot be solved by adjusting the drawing tension as described in JP-A Nos. 07-234322 and 07-234324, so it is required to keep the transmission loss while the rolled POF is stored.
An object of the present invention is to provide a method to manufacture a plastic optical fiber with low transmission loss, capable of controlling the transmission loss of the POF while the POF is stored for a long time.
Another object of the present invention is to provide a plastic optical fiber manufactured by such manufacturing method.